This invention relates to solid-state membrane modules formed from membrane units which are capable of separating a gaseous species from a gaseous species-containing gaseous mixture. This invention further relates to solid-state membrane modules formed from membrane units which are capable of separating oxygen from an oxygen-containing gaseous mixture. The modules, which provide improved pneumatic integrity, may be fabricated from a plurality of planar solid-state membrane units comprising mixed conducting metallic oxides which exhibit electron conductivity and oxygen ion conductivity at elevated temperatures.
Solid state membranes formed from oxygen ion-conducting materials continue to show promise in a variety of commercial processes including the separating of oxygen from oxygen-containing gaseous mixtures. Representative solid-state membranes are those formed from multicomponent metallic oxides which are typically operated at high temperatures (e.g. 700° C. or more) wherein the solid-state membranes conduct both oxygen ions and electrons. When a difference in oxygen partial pressure exists on opposite sides of the mixed conducting metallic oxide membrane and operating conditions are properly controlled, oxygen is separated from the oxygen-containing gaseous mixture as oxygen ions migrate to the low oxygen partial pressure side of the solid-state membrane while an electron flux occurs in the opposite direction of oxygen ion migration in order to conserve charge, producing pure oxygen on the permeate side of the membrane.
Alternatively, the permeated oxygen can be reacted directly with a hydrocarbon-containing gas, either catalytically or non-catalytically, to yield a hydrocarbon oxidation product. Various oxygen-containing gases, such as air, can be used and numerous alternative hydrocarbon oxidation products are possible depending on the operating conditions and catalyst if used.
There is a significant and growing commercial interest in the production of synthesis gas from natural gas and air using mixed-conducting ceramic membrane reactor systems. This technology is presently in the development stage and commercial applications are envisioned in future years as the technology matures. Mixed-conducting ceramic membrane reactor systems produce synthesis gas by the partial oxidation methane to form the synthesis gas components CO, H2, CO2, and H2O. The process is carried out by introducing a methane-containing feed gas and an air feed gas into the membrane reactor system, contacting one surface of the membrane with methane, and contacting the other surface with air. Oxygen permeates through the membrane, methane reacts with permeated oxygen to form a methane/synthesis gas mixture, and methane is further converted into synthesis gas as the mixture travels through the reactor while reacting with additional permeated oxygen.
This process can be integrated favorably with upstream and downstream processes if the methane/synthesis gas stream is at a high pressure, typically 250-450 psig. In addition, process economics are most favorable if the air is at a low pressure, typically less than 50 psig. Therefore, the membranes in the membrane reactor system must be designed to withstand a significant pressure differential between the air side and the methane/synthesis gas side. To achieve high oxygen fluxes through the membrane, the active separating layer of the membrane should be thin, typically less than 200 microns. However, a freestanding membrane of this thickness would have difficulty withstanding a typical pressure differential of 200-400 psig, and the thin separating layer therefore may be structurally supported in some fashion.
A plurality of solid state membrane units may be joined together to form a membrane module wherein passageways are incorporated between each respective membrane unit in order to facilitate introducing the oxygen-containing gaseous mixture to be separated into the module and recovering the oxygen product from the module.
Gas separation modules and fuel cells of the prior art are typically operated under conditions such that a near zero pressure differential exists across the membrane cells wherein problems associated with pneumatic integrity are minimized and minor leaks are tolerated to a limited extent between the cells. These modules may be manifolded in a configuration such that oxygen can exit through the channels within each membrane unit.
Industry is searching for solid-state membrane modules which are suitable for conducting a wide variety of processes and reactions wherein the modules would exhibit improved pneumatic and structural integrity. Moreover, such modules would desirably be readily fabricated and manifolded and would be capable of withstanding the pressure differential necessary in practicing air separation processes and desirable in practicing partial oxidation processes.